Article pubs.acs.org/JPCA

Glass Formation Processes in Mixed Inorganic/Organic Aerosol Particles Hans P. Dette and Thomas Koop* Faculty of Chemistry and Center for Molecular Materials, Bielefeld University, Universitätsstraße 25, D-33615 Bielefeld, Germany S Supporting Information *

ABSTRACT: Recent experiments suggest that organic aerosol particles may transform into a glassy state at room temperature under dry conditions. Information on glass forming processes in mixed inorganic/organic aerosol particles is sparse, however, because inorganic crystal nucleation is usually very likely in such mixtures. Here we investigate the glass transition temperatures Tg of various organics (trehalose, sucrose, citric acid, sorbitol, and glycerol as well as 3-MBTCA) in binary mixtures with either NaNO3 or NH4HSO4 at different mass fractions. The glassy samples were prepared with the MARBLES technique by atomizing dilute aqueous solutions into aerosol particles and subsequent diffusion drying. The resulting aerosol particles were collected and their phase behavior was investigated using differential scanning calorimetry. At small and intermediate inorganic mass fractions salt crystallization did not occur. Instead, the single-phase mixtures remained in an amorphous state upon drying such that determination of their Tg was possible. From these measurements the Tg value of pure NaNO3 and pure NH4HSO4 could be inferred through extrapolation, resulting in values of Tg(NaNO3) ≈ 290 K and Tg(NH4HSO4) ≈ 220 K. Upon drying of NH4HSO4/3-MBTCA mixtures, phase-separated samples formed in which the inorganic-rich and organic-rich phases each show an independent glass transition. Our measurements provide a route toward establishing Tg values of inorganic salts that usually crystallize readily, and they may explain the reported contradicting observations of NaNO3 aerosol particles to either crystallize or remain amorphous upon drying at room temperature.

1. INTRODUCTION The glassy state of matter is an area of active research in terms of our fundamental understanding of condensed phases but also in many practical applications. For example, glasses are important in materials science, cryobiology, food sciences, pharmaceutical research, and atmospheric aerosol science.1−8 Considerable progress has been made over the last decades in understanding the properties and behavior of glassy materials, but many unknowns still do exist. For example, describing properties of the glassy state and predicting the glass transition temperature Tg of binary and multicomponent mixtures of molecular and ionic compounds is still poorly understood.8,9 A substantial number of data on Tg of single organic compounds and of mixtures of organic compounds exist.8,10−17 In addition, Tg of many aqueous solutions of glass-forming organics have been investigated, in particular in the food and pharmaceutical sciences and also in atmospheric contexts, but only a few of these studies contain data on aqueous solutions of mixed inorganic/organic solutes, especially toward the dry state.5,7,9,18−20 Moreover, Tg data for inorganic electrolytes and salts are rather limited, although some data do exist as well as data of aqueous solution mixtures or in salt mixtures.13,19,21−24 Often, such measurements are hampered by the fact that most salts crystallize readily when they are cooled below their melting point or when aqueous solution samples are dried in both pure and mixed compounds.25−27 Nonetheless, knowledge of Tg of pure inorganic compounds may help for a better description of efflorescence behavior, ice nucleation ability, © XXXX American Chemical Society

water uptake kinetics, condensed-phase chemical reaction kinetics, and the glass-forming properties of multicomponent (e.g., inorganic/organic) aerosol particles.28−41 In this paper we use the MARBLES technique that we developed recently42 for studying Tg of dry inorganic/organic mixtures. The MARBLES technique involves vitrification by drying of aerosol particles, which are advantageous for preparing glasses, because their small sample size reduces the chance of crystallization considerably. The size of the aerosol particles investigated with MARBLES is very similar to that of aerosol particles occurring in the atmosphere. Atmospheric aerosols are the topic of a number of fundamental research questions because of their effects in Earth’s atmosphere, climate, and health,43 adding to the interdisciplinary political and societal discussion on climate change issues.44 Atmospheric aerosol particles are often internal mixtures of organic compounds and inorganic electrolytes such as sulfates and nitrates.45−51 Recently, it has been shown that aqueous solutions droplets or water-soluble organics of atmospheric relevance can form semisolid and glassy states.8,9,19,52 In addition, evidence for such an amorphous (semi)solid state of atmospheric aerosol particles have also been found in field measurements in boreal forests.53 Since Special Issue: Mario Molina Festschrift Received: October 24, 2014 Revised: December 9, 2014

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adjustment is done because of small ambiguities in the evaluation of the slopes and the repeatability of the experimentally determined Tg values across multiple samples. The height of a glass transition step in the thermograms, hTg, stemming from the change in the heat capacity at Tg, was determined by the difference of the heat flow 5 K below and above the glass transition temperature, which is defined here as the point of largest slope in heat flow. The magnitude of a crystallization signal was determined as the peak area using a sigmoidal baseline interpolation. If two peaks overlapped, for example during consecutive crystallization events, the local minimum between the two signals was used as the end point or starting point of a peak, respectively. 2.2. MARBLES method. We have used the MARBLES method (metastable aerosol by low temperature evaporation of solvent), described in detail in a previous publication,42 for preparing glassy samples of water-soluble substances. Briefly, in MARBLES a dilute aqueous solution with solute mass fraction w(sol) = 0.02 is atomized into an aerosol that is dried subsequently by means of several consecutive diffusion dryers. The dried particles are collected in an aluminum DSC sample pan via a homemade impactor until enough mass has accumulated for a DSC measurement (about 25 mg, corresponding to a collection time of 2040 min). The sample pan is sealed with a crimper press and is investigated thereafter by means of differential scanning calorimetry. The sample collection and sealing are done at room temperature inside a glovebox under dry conditions with a relative humidity of 0% ± 0.1% RH to avoid sample contamination by water from the laboratory atmosphere. For investigations of the glass transition of binary solute mixtures, the two substances were mixed in the desired mass ratio and dissolved in water so that the overall mass fraction was 2%, i.e., w(sol1+sol2) = 0.02. 2.3. Materials. All aqueous solutions were prepared with bidistilled deionized water. As a drying agent in the MARBLES process we used Silica Gel Orange (Roth, diameter 2−5 mm, specific surface area 750 m2 g−1). Except for 3-MBTCA, all organic and inorganic substances were obtained commercially. NH4HSO4 (99.99%), sorbitol (≥98%), and trehalose dihydrate (≥99%) were obtained from Sigma-Aldrich, NaNO3 (≥99.5%) from Merck, sucrose (Ph. Eu.) and glycerol (≥98%) from Roth, and citric acid (99.8%) from Acros Organics. 3-Methylbutane1,2,3-tricarboxylic acid was synthesized and characterized by ourselves as described in detail in a previous publication.42 2.4. Fitting Procedure. The glass transition temperature Tg in a binary mixture can be estimated with the semiempirical Gordon−Taylor equation74

then, indirect evidence for semisolid or glassy states in proxies for atmospheric secondary organic aerosol (SOA) particles have been provided in a number of cases. For example, in previous laboratory experiments, the effects of glass formation in mixed inorganic/organic droplet samples have been studied. But instead of determining Tg, these experiments often indirectly infer the presence of glassy or semisolid states of high viscosity by studying water uptake kinetics (e.g., amorphous deliquescence), or the kinetics and location of crystal nucleation (e.g., salt efflorescence or heterogeneous ice nucleation).28−34,54 On the one hand, it has been observed that such inorganic/organic aerosol particles are in a well-mixed single-phase state when water is part of the mixture and the atomic O:C ratio of the organic compounds is larger than about 0.8. On the other hand, such mixtures may result in liquid− liquid phase-separated particles at lower O:C ratios. In these cases one phase is enriched in organic solutes while the other is enriched in inorganic solutes.55−61 Here, we use MARBLES to investigate the two salts NaNO3 and NH4HSO4 in mixtures with glass-forming organics. The Tg values of these two salts are not well established in their pure state.9,19,21,22,62,63 We employ the binary compound systems to determine Tg of the mixtures themselves, but more importantly also as a means to establish approximate values for Tg of the pure compounds by extrapolation. We chose NaNO3 because it may occur in atmospheric aerosols in coastal regions64 and in mixed maritime/polluted air masses.65 NH4HSO4 was chosen because it may resemble partially neutralized sulfates frequently found in atmospheric aerosol particles and employed in modeling studies.51,66−72 The paper is structured as follows: After a brief description of the experimental procedures, we present results of measured glass transition temperatures of homogeneous (one-phase) mixtures of two inorganic substances, NaNO3 and NH4HSO4, with various organic substances each. From these data we infer the glass transition temperatures of the two salts in their pure state. Finally, we present results of a heterogeneous (twophase) mixture of NH4HSO4 and 3-methylbutane-1,2,3tricarboxylic acid (3-MBTCA), in which each phase undergoes an individual distinguishable glass transition.

2. EXPERIMENTAL, METHODS, AND MATERIALS 2.1. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) is a well-established method for investigating phase changes such as crystallization, melting, and glass formation. For this study we used a differential scanning heat-flow calorimeter (Q100, TA Instruments) with a temperature accuracy of about ±0.2 and ±0.4 K at heating rates of 1 and 10 K min−1, respectively. Calibration was done using a series of nine calibration standards at cooling and heating rates between −10 and +10 K min−1.73 In our measurements about 1−10 mg of a sample was collected by means of aerosol particle impaction in an aluminum pan placed inside an impactor (thereafter sealed hermetically) and then transferred into the calorimeter. An empty pan was used in these measurements as a reference. The glass transition temperature Tg of a sample was determined in the heating mode at 10 K min−1 using the onset of the glass transition signal following the convention of Angell,2 i.e., the intersection between the extrapolated baseline and a line of maximum slope in the heat flow signal. Despite the calorimeter’s temperature accuracy of ±0.4 K for this heating rate, we estimate the accuracy of the glass transition temperatures given in this study to be about ±2 K. This

1

Tg =

w1Tg1 + k w2Tg2 1

w1 + k w2

(1)

Here, Tg is the glass transition temperature of the binary mixture, Tg1 and Tg2 are the glass transition temperatures of the two pure compounds, w1 and w2 are the mass fractions of the two compounds in the mixture, and k is the Gordon−Taylor constant. In the following analysis of various binary systems we assign compound 1 to the inorganic salt, i.e., in this case Tg1 is the unknown and sought-after glass transition of the pure inorganic substance, and compound 2 is a glass-forming organic substance with known Tg2. In the measurements presented below, we performed Tg measurements for several binary mixtures consisting of the same salt (substance 1) and different B

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3. RESULTS 3.1. Homogeneous (One-Phase) Inorganic/Organic Mixtures. Tg of NaNO3/Organic Mixtures. We investigated the glass transition temperatures of NaNO3 in binary mixtures with four different organic compounds: trehalose, sucrose, citric acid, and sorbitol. We chose these substances because they are known to be good glass formers and their Tg in the pure state and in some mixtures are well described in the literature.7,11,12,75−80 We used the MARBLES method to transfer various samples of each of the binary mixtures into an amorphous dry state and subsequently investigated their Tg using the differential scanning calorimeter. It was possible to determine Tg values of the different binary mixtures in a range of NaNO3 dry mass fraction w(NaNO3) of between 0−0.7 and 0−0.8, depending on the organic compound. Glassy samples with higher NaNO3 mass fractions could not be obtained because such mixtures crystallized upon drying during the MARBLES process, as described later. Figure 1 shows exemplary several DSC thermograms of NaNO3/sorbitol mixtures. The glass transition signal in the thermograms shifted continuously toward higher temperature with increasing NaNO3 mass fraction (Figure 1a), approaching the estimated glass transition temperature of pure NaNO3 of about 290 K; see analysis below. Typically, crystallization of the sample occurred upon further heating beyond the glass transition temperature; see Figure 1b. This signal is most likely due to the formation of crystalline NaNO3 because the peak area becomes smaller with decreasing NaNO3 content and the peak is entirely absent in the pure sorbitol sample with w(NaNO3) = 0. Note that pure sorbitol also does not crystallize below its melting temperature of ∼368 K, making it a very good glass former. In samples with w(NaNO3) ≥ 0.8 homogeneous amorphous onephase particles did not form because crystallization of NaNO3 occurred already during sample preparation, so that the glass transition signal of the mixture at the corresponding temperature is absent and only a very small crystallization peak was observed (not shown). The thermograms of the NaNO3/ trehalose mixtures are very similar as they also show a continuous shift toward the Tg of pure NaNO3 with increasing NaNO3 mass fraction. However, upon further warming beyond Tg two separate exothermic peaks are observed in these thermograms (Figure 1c). Two such exothermic peaks occur when both compounds crystallize separately upon heating. The relative change in peak area of the two peaks with changing NaNO3 dry mass fraction in the mixture allows the assignment of the low temperature peak to NaNO3 crystallization and the high temperature peak to trehalose crystallization (Figure 1c). Figure 2 shows all glass transition temperatures of the four investigated mixtures. For trehalose, sucrose, and sorbitol the Tg data (circles) show a constant trend and can be fitted well with eq 1 (solid lines). But the binary NaNO3/citric acid system behaves somewhat differently. The observed glass transition temperatures first increase from 286 K for pure citric acid, i.e., w(NaNO3) = 0, to 297 K for w(NaNO3) = 0.3. For higher NaNO3 mass fractions the Tg values decrease and approach Tg of pure NaNO3 as obtained from the extrapolation of the three other binary systems investigated here. We suggest that this behavior is due to adduct formation corresponding to

Figure 1. DSC thermograms of mixed NaNO3/organic samples: (a) Glass transition signals of NaNO3/sorbitol mixtures for various w(NaNO3); (b) NaNO3 recrystallization in NaNO3/sorbitol mixtures at temperatures above Tg; (c) recrystallization in NaNO3/trehalose mixtures above Tg (inset depicts the corresponding Tg signals). Heat flow baselines were shifted for clarity in all panels. An exothermic process results in a positive heat flow signal.

the monosodium citrate (NaH2Cit). In a very recent publication Wang and Laskin41 used an experimental approach similar to ours for studying the properties of aerosol particles nebulized from aqueous NaNO3/citric acid solutions with a molar ratio of 1:1, i.e., w(NaNO3) = 0.307. Upon diffusion drying of the aerosol amorphous and homogeneous particles formed in agreement with our DSC measurements showing one single Tg signal. Moreover, from quantitative EDX they inferred a 50% loss of nitrogen relative to sodium from the dried particles. They argued that this loss is due to nitrate depletion caused by a partial reaction of citric acid with NaNO3, leading to the formation and evaporation of HNO3 from the particles. The remaining particles then formally have a 1:1:1 stoichiometry of citric acid, NaNO3, and NaH2Cit. Such a stoichiometry is consistent with the maximum in Tg observed in our study at w(NaNO3) ≈ 0.3 because the glass transition of NaH2Cit of Tg = 342 K81 is larger than that of both citric acid C

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extrapolations of Tg in binary mixtures of Cd(NO3)2·4H2O and NaNO3 in the eutectic concentration range, thus requiring a significant extrapolation.21 Also a theoretical estimate from an extrapolation of heat capacity data of the liquid and crystalline state to low temperatures results in an estimate of about 216 K.21 Moreover, data from NaNO3 thin films prepared by vapor deposition yielded a Tg below 224 K.62,63,82 However, the NaNO3 vapor was obtained through expansion of the gas phase over a molten NaNO3 sample. The vapor composition was not determined, although the absence of N2O4 as a decomposition product of nitrate was checked. Nevertheless, this preparation method may result in some uncertainty regarding the thin films’ stoichiometry and composition and, hence, also in the resulting Tg of NaNO3. We do not have a definite explanation for the difference between previous Tg estimates of NaNO3 and that reported here. But we think that our approach of determining Tg is more reliable because of two arguments. First we have covered a wider range of NaNO3 mass fractions in several independent binary mixtures, resulting in a better stability of the Tg extrapolation to pure NaNO3. Second, we observed a clear upward trend for the Tg values in NaNO3/sorbitol mixtures up to w(NaNO3) = 0.7. This trend strongly suggests that the Tg of NaNO3 is higher than the corresponding Tg of 282 K at w(NaNO3) = 0.7. Moreover, we note a similar discrepancy of reported Tg values of another alkali nitrate, LiNO3. More recent data also support a much higher Tg of LiNO3 of about 280 K83,84 (Austen Angell, personal communication) instead of 228 K reported in previous work.21 Finally, a Tg of NaNO3 close to room temperature is also in agreement with a number of seemingly contradictory observations of the efflorescence in aqueous NaNO3 droplets upon drying: in some measurements such droplets could be dried without crystallization and, hence, transferred into an amorphous dry state.26,41,85−87 In others, crystallization occurred upon drying.85,86,88 In particular, the latter studies showed that nucleation of crystalline NaNO3 can be triggered heterogeneously, for example, by impurities or the surface of a substrate, whereas in freely suspended droplets or on inert surfaces, crystal nucleation could be avoided and a dry amorphous state of NaNO3 resulted. Similarly, addition of glycerol to aqueous NaNO3 droplets also let to amorphous solid formation rather than efflorescence.89 All these data support a Tg close to room temperature: On the one hand, if Tg of NaNO3 was indeed at about 230 K as suggested previously it appears unlikely that NaNO3 crystal nucleation can be avoided at room temperature, i.e., more than 60 K above Tg and about 290 K below Tm. On the other hand, if Tg of dry NaNO3 is close to room temperature as suggested here, then NaNO3 crystallization may not occur as long as heterogeneous nucleation is avoided during drying of supersaturated aqueous NaNO3 solution droplets. Tg of NH4HSO4/Organic Mixtures. The investigations of the glass transition temperature of NH4HSO4 and its mixtures with organic compounds were performed in a manner analogous to those of NaNO3. Here, we chose trehalose, citric acid, and glycerol as organic compounds. Again, a continuous shift of the Tg signal with increasing NH4HSO4 mass fraction was observed in mixtures with citric acid and with trehalose. Figure 3a shows this shift of the Tg signal in thermograms of NH4HSO4/citric acid samples with different NH4HSO4 mass fractions. The data for NH4HSO4/trehalose are similar (not shown).

Figure 2. Overview of measured Tg values as a function of NaNO3 mass fraction in four NaNO3/organic mixtures. The solid and dashed lines represent two different fitting procedures, one including the Tg of the pure compounds indicated by colored open circles (solid lines) and one without them (dashed lines). Both fits result in a very similar extrapolation toward Tg of pure NaNO3 (open gray circles). The open blue squares indicate data points that were excluded from the citric acid data fit (dashed blue line); for details see the text.

and NaNO3. Quite generally, such a local Tg maximum with varying composition cannot be described by the Gordon− Taylor eq 1. Nevertheless, it is expected that Tg of the mixtures approach the Tg of pure NaNO3 when the mass fraction of NaNO3 becomes so large that it dominates the glass forming behavior of the sample. This trend is indeed observed for NaNO3 mass fractions larger than 0.3 (solid blue circles in Figure 2). Interestingly, if these data points of mass fractions ≥0.3 are fitted with the Gordon−Taylor equation, an extrapolation to w(NaNO3) = 0 results in a Tg2 value of about 310 K. This value is between those of pure citric acid Tg(H3Cit) = 286 K and monosodium citrate Tg(NaH2Cit) = 342 K, thus providing indirect support that the samples for w(NaNO3) ≥ 0.3 can be represented as mixtures of NaNO3 and NaH2Cit/H3Cit, in agreement with Wang and Laskin.41 For the determination of the Tg of pure NaNO3 by extrapolation to w(NaNO3) = 1 in the different binary mixtures we used two alternative fitting procedures indicated by two different line styles in Figure 2. First, all data of the three binary systems with trehalose, sucrose, and sorbitol (all circles) were fitted simultaneously with eq 1 (solid lines), resulting in a Tg value for pure NaNO3 of 293 ± 2 K. Adduct formation between NaNO3 and the polyols is unlikely because of the absence of highly reactive groups such as carboxylic groups. Nonetheless, comparing the fitted lines to the experimental data indicate that the Tg data of the pure organic substances (open circles) differ a little bit more from the fitted lines than the rest of the data (solid circles), in particular for trehalose. When the Tg values of the pure organics at w(NaNO3) = 0 are neglected in a second fitting procedure (dashed lines), the extrapolated Tg for NaNO3 is only somewhat lower at 287 ± 2 K. In this fit we included also the data of NaNO3/citric acid mixtures for w(NaNO3) ≥ 0.3 (solid blue circles). In conclusion, the extrapolated Tg resulting from the two fitting procedures agree within a few kelvins. Taking both fits into account, we suggest that the Tg of pure NaNO3 is 290 ± 5 K. The result of a Tg ≈ 290 K for pure NaNO3 comes rather surprising, because previous estimates suggested a value closer to about 225 K.21,22,62,63 These Tg values result from D

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Figure 4) and those at higher mass fractions are Tg values of two-phase mixtures containing NH4HSO4 crystals in a saturated glycerol solution (green crosses).

Figure 4. Overview of measured Tg values (solid circles) as a function of NH4HSO4 mass fraction in three NH4HSO4/organic mixtures. The solid lines represent results from fitting all data simultaneously (see text) and their extrapolation toward Tg of pure NH4HSO4 (open gray circle). The green crosses are data of samples in which NH4HSO4 crystallized during the MARBLES process in agreement with Tg of a saturated bulk solution of NH4HSO4 in glycerol (dotted green line). Figure 3. DSC thermograms of mixed NH4HSO4/organic samples. (a) Tg signals of NH4HSO4/citric acid mixtures for various w(NH4HSO4). (The small peak in the bottom thermogram is a measurement artifact.) (b) Tg signals of pure glycerol (black) and of two NH4HSO4/glycerol mixtures with w(inorg) ≥ 0.3 (red and blue), and of a saturated bulk solution of NH4HSO4 in glycerol (magenta). Dotted vertical lines indicate the Tg of pure glycerol Tg,glyc (black) and the Tg of the saturated solution Tg,sat (magenta). The thin solid lines visualize the onset of the glass transition signals used to determine Tg. Heat flow baselines were shifted for clarity in both panels.

Fitting the Tg data of the NH4HSO4/glycerol system with inorganic mass fractions ≤0.3 together with the data of the other two investigated systems using eq 1 results in an extrapolated glass transition temperature for pure NH4HSO4 of about Tg = 222 K ± 4 K (Figure 4). This Tg value seems to be in contrast to that of previous work by Zobrist et al.19 of Tg ≈ 178 K. In that work, aqueous NH4HSO4 solutions with NH4HSO4 mass fractions of about 0.50.8 were investigated, resulting in Tg values of about 153168 K, respectively. From these data points and the Tg of pure water (136 K) they extrapolated a Tg of pure NH4HSO4 of 178 K with the Gordon−Taylor equation. In contrast, our data from three independent binary systems (Figure 4) yield an extrapolated Tg of about 222 K. More importantly we approach Tg of NH4HSO4 from above (trehalose and citric acid) and below (glycerol). Particularly, the upward trend of the Tg values for the NH4HSO4/glycerol mixtures is strongly suggestive of a Tg of pure NH4HSO4 that is larger than 195 K. Also, adding a Tg of 222 K to the data of Zobrist et al. results in a Gordon− Taylor fit with about the same correlation coefficient as the fit to the original data without this value. Moreover, when we include the NH4HSO4/H2O data by Zobrist et al. to our fitting procedure, a Tg of pure NH4HSO4 of about 219 ± 6 K results (see Figure S1 in Supporting Information). This analysis indicates that there is no contradiction between our data and those of Zobrist et al., but instead they are consistent with each other. We thus suggest that the glass transition temperature of pure NH4HSO4 is assigned to a value of Tg ≈ 220 K. Finally, we note that we did not analyze the type of crystals that formed during drying of the most concentrated NH4HSO4/organic mixtures, because it is not important for the Tg determination whether indeed pure NH4HSO4 crystallized or instead letovicite, (NH4)3H(SO4)2, a crystalline phase that is often observed in efflorescence and freezing studies of aqueous NH4HSO4 solutions, is formed.25,66,91−94

The NH4HSO4/glycerol mixtures, however, behaved somewhat differently. In this binary system, Tg first increases from about 190 K for pure glycerol to about 194.5 K for w(NH4HSO4) = 0.3. But for higher NH4HSO4 mass fractions Tg does not increase any further but instead remains constant at about 194.5 K. A constant Tg may be indicative of a constant composition of the glassy phase. But because the overall composition of the samples varied, the observation of constant composition thus suggests a partial crystallization of NH4HSO4 during the MARBLES procedure. At room temperature the viscosity of pure liquid glycerol is on the order of 1 Pa s,90 thus allowing for a rather fast establishment of the crystalline NH4HSO4/solution equilibrium. To test the hypothesis of NH4HSO4 crystallization, we prepared a bulk solution of NH4HSO4 in glycerol by adding crystalline NH4HSO4 to a few milliliters of glycerol and stirring this mixture for about 24 h at room temperature. Crystalline NH4HSO4 remained during the entire time period, indicating the formation of a saturated solution (concentration was not determined). At the end a few milligram of this saturated solution was added directly to a DSC pan and investigated thereafter. This saturated bulk solution of NH4HSO4 in glycerol showed the same Tg of 194.5 K as the mixtures with dry mass fractions larger than 0.3 prepared with MARBLES (Figure 3b), clearly supporting our hypothesis that NH4HSO4 crystallization occurred in MARBLES. Hence, only those Tg values with w(NH4HSO4) ≤ 0.3 can be considered to represent Tg of one-phase mixtures (solid green circles in E

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The Journal of Physical Chemistry A 3.2. Liquid−Liquid Phase-Separated (Two-Phase) Inorganic/Organic Mixtures. Tg of NH4HSO4/3-MBTCA Mixtures. We also investigated mixtures of 3-methylbutane1,2,3-tricarboxylic acid (3-MBTCA) with NH4HSO4. 3MBTCA is an oxidation product of α-pinene and a common marker compound in secondary organic aerosol particles occurring in boreal forest areas.47,95−98 Therefore, these NH4HSO4/3-MBTCA mixtures probably represent the most realistic type of proxy for atmospheric inorganic/organic aerosols of the mixtures investigated in this study. The Tg measurements for this system were done analogous to the experiments described above by drying an aqueous solution of known NH4HSO4/3-MBTCA dry mass ratio. Figure 5 shows a

inorganic mass fraction led to a continuous change in the Tg value of the mixture. In contrast, varying the NH4HSO4 mass fraction in the NH4HSO4/3-MBTCA mixtures did not lead to a significant change in either of the two measured Tg signals (Figure 6a). This observation suggests a constant composition

Figure 6. (a) Tg data (red and blue circles) of phase-separated NH4HSO4/3-MBTCA mixtures as a function of the overall NH4HSO4 mass fraction. Dotted lines represent the measured Tg of pure 3MBTCA (red) and the extrapolated Tg of pure NH4HSO4 (blue). (b) Difference ΔhTg between the glass transition signal heights hTg1 and hTg2 for various overall NH4HSO4 mass fractions.

Figure 5. Thermogram of an NH4HSO4/3-MBTCA mixture with w(NH4HSO4) = 0.5. Two Tg signals (highlighted by blue and red boxes) are observed, which belong to two phase-separated amorphous phases with different composition. At higher temperature their respective recrystallization and melting peaks are observed.

of the two phases, just as expected for a phase-separated mixture, in which the fixed composition of the two phases is determined by thermodynamics.61,102 The calorimetric measurements allow concluding further details about the nature of the two phases. Our observations of two glass transition signals implies that both phases do not crystallize upon complete drying but instead remain in an amorphous state. The observed temperatures at which the two glass transitions occur are Tg1 ≈ 233 K and Tg2 ≈ 304 K (blue and red points in Figure 6a). Hence, the phase with the low Tg value can be assigned to a phase rich in NH4HSO4 because Tg1 is only 13 K above that of pure NH4HSO4 determined above (220 K, blue dashed line in Figure 6a). Furthermore, the second glass transition signal at about Tg2 ≈ 304 K is practically identical to the Tg value of pure 3-MBTCA obtained in a previous study42 (305 K, red dashed line in Figure 6a). We conclude that the organic-rich phase is practically pure 3MBTCA whereas the inorganic phase is dominated by NH4HSO4 but also included some 3-MBTCA. Though the composition of the two phases in a phaseseparated mixture is constant, an increase in the overall NH4HSO4 mass fraction should result in a larger mass of the inorganic-rich phase relative to that of the organic-rich phase. This trend is indeed supported by our observations. The Tg signal height hTg of the inorganic-rich phase (Tg1 ≈ 233 K) becomes larger relative to that of the organic-rich phase (Tg2 ≈ 304 K) as the difference between the two heights ΔhTg = hTg1 − hTg2 increases continuously with increasing NH4HSO4 mass fraction (Figure 6b). Similar arguments hold for the two crystallization signals upon warming just beyond Tg2. The crystallization signal assignment to the two phases can be done

typical thermogram for w(NH4HSO4) = 0.5. In contrast to the other mixtures discussed above, two glass transition signals can be observed, one at about 233 K and another at about 303 K, the latter directly followed by two overlapping crystallization signals. Observation of two glass transitions in a single DSC thermogram is possible, if a sample consists of two separate phases with different compositions and, hence, also different glass transition temperatures. Such measurements have been described already for nonatmospheric systems such as polymer mixtures or aqueous starch−glycerol mixtures in which the occurrence of two glass transitions was also assigned to the presence of two separate phases within the mixture.99−101 The presence of two phases is possible in the NH4HSO4/3MBTCA system if it undergoes a phase separation upon drying in the MARBLES process, resulting in a NH4HSO4-rich phase and another 3-MBTCA-rich phase. This phase separation behavior upon drying is known to occur for a number of inorganic/organic mixed systems and is well described in the literature.59−61 It has been shown that phase separation occurs upon drying and depends on the O:C ratio of the organic compound.55−58 For low O:C ratios ≤0.5 the inorganic/organic mixtures always showed phase separation whereas at high O:C ratios ≥0.8 phase separation never happened, and for O:C ratios between these values phase separation happened in some cases but not in all. With an O:C ratio of 0.75, 3-MBTCA falls in this intermediate range where phase separation may occur, which is consistent with our observations of two glass transitions. Further support comes from the results of experiments in which the NH4HSO4 mass fraction was varied. In the one-phase mixtures described above a change in F

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NH4HSO4 occurs again about 60 K above the observed glass transition temperature.

in the same manner as described for the trehalose/NaNO3 system. The ratio of the peak area of the two signals change in line with the NH4HSO4/3-MBTCA mass ratio of the sample (Figure 7) If the mass fraction of NH4HSO4 is larger in the

4. SUMMARY AND CONCLUSION In this paper we have presented measurements of the glass transition temperature of a number of mixed inorganic/organic binary systems. The samples were vitrified using the MARBLES technique in which aqueous aerosol particles are transferred into a glassy state by means of diffusion drying. In particular, Tg measurements of mixtures of several glass forming organic compounds with either NaNO3 or NH4HSO4 were performed. Even though at the largest inorganic mass fractions the salts crystallized in each of the investigated samples, the different binary systems otherwise remained amorphous for a wide range of mass fractions. The Tg value of pure NaNO3 or pure NH4HSO4 could be inferred through simultaneous extrapolation of the various binary systems, yielding values of Tg(NaNO3) ≈ 290 K and Tg(NH4HSO4) ≈ 220 K. In particular, the Tg value of NaNO3 obtained here is larger than previous suggestions, but it is consistent with a number of single droplet efflorescence experiments. Moreover, similar differences are reported in the literature for another alkali nitrate, LiNO3. We note that even if the extrapolated Tg values may overestimate the actual Tg of the pure compounds, they may serve as very helpful parameters for the description of how these salts influence Tg in mixtures and thereby parametrize the behavior of inorganic/organic mixtures. We also presented the first measurements of two independent glass transitions in phase-separated NH4HSO4/ 3-MBTCA samples. We suggest that a similar behavior may also occur in atmospheric aerosol particles consisting of mixtures of secondary organic materials and partially neutralized sulfates. We are currently working on measurements of the Tg of other inorganic substances of atmospheric relevance such as (NH4)2SO4 or NH4NO3, but these measurements are not as straightforward as the results presented here. In preliminary experiments (NH4)2SO4 crystallized readily in binary mixtures with various organics even at medium mass fractions. Hence, (NH4)2SO4 requires large amounts of organics to prevent crystallization and allow glass formation, which in turn decreases the accuracy of the extrapolation toward pure (NH4)2SO4, leading to a less precise Tg value. It is wellknown that NH4NO3 shows significant volatility at room temperature.103 Hence, it is possible that the NH4NO3 content of mixed NH4NO3/organic aerosol particles is altered during the drying procedure, requiring those experiments with MARBLES to be carried out at lower temperature.

Figure 7. DSC thermograms of selected NH4HSO4/3-MBTCA mixtures showing two recrystallization peaks at temperatures above Tg2. Heat flow baselines were shifted for clarity.

original mixture, the glass transition step of the NH4HSO4-rich phase is more pronounced than that of the organic-rich phase and, accordingly, also the NH4HSO4 crystallization signal is larger than that of 3-MBTCA (Figure 7). Finally, we note another peculiarity of the NH4HSO4/3MBTCA system. The NH4HSO4-rich phase remains in an amorphous state upon drying at room temperature even though it is prepared about 60 K above its glass transition. Many other samples crystallize when the temperature is so much higher than Tg. The fact that crystallization does not occur in the investigated mixture indicates that 3-MBTCA is very effective at inhibiting NH4HSO4 crystallization. As an additional test we prepared another sample in which we further increased the NH4HSO4 mass fraction to 14 parts NH4HSO4 in only 1 part 3-MBTCA corresponding to w(NH4HSO4) = 0.93. When this sample is dissolved and dried with MARBLES, it remains amorphous and shows a single glass transition at around 234 K (Figure 8), whereas no Tg signal close to that of a potential

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ASSOCIATED CONTENT

S Supporting Information *

A variant of Figure 4 that includes Tg values of NH4HSO4/H2O mixtures from the literature is presented in supplementary Figure S1. The measured glass transition temperatures shown in Figures 2 and 4 are presented in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Thermogram of an NH4HSO4/3-MBTCA mixture with w(NH4HSO4) = 0.93. Only one Tg signal is observed.

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organic-rich phase could be detected, suggesting that w(NH4HSO4) = 0.93 is very close to the miscibility gap of the NH4HSO4/3-MBTCA system at room temperature and is close to the composition of the inorganic-rich phase in the twophase samples. Figure 8 also shows that crystallization of

AUTHOR INFORMATION

Corresponding Author

*T. Koop. E-mail: [email protected]. G

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The Journal of Physical Chemistry A Author Contributions

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The study was conceived by H.P.D. and T.K., the experiments were performed by H.P.D., results were analyzed by H.P.D. and TK., and the paper was written by H.P.D. and T.K. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mian Qi, David Schröder, and Adelheid Godt for essential contribution during the synthesis of 3-MBTCA. Furthermore, we thank Austen Angell for helpful discussions about Tg data of mixtures containing NaNO3 or LiNO3. H.P.D. acknowledges a Nachwuchsfonds stipend from Bielefeld University.

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ABBREVIATIONS DSC differential scanning calorimetry EDX energy disperse X-ray spectroscopy hTg height of a glass transition step in a thermogram H3Cit citric acid MARBLES metastable aerosol by low temperature evaporation of solvent 3-MBTCA 3-methylbutane-1,2,3-tricarboxylic acid NaH2Cit monosodium citrate O:C ratio oxygen-to-carbon atom ratio of an organic substance RH relative humidity SOA secondary organic aerosol Tg glass transition temperature Tm melting temperature

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DOI: 10.1021/jp5106967 J. Phys. Chem. A XXXX, XXX, XXX−XXX